Method and apparatus for cathodic protection

ABSTRACT

An improved apparatus and method for cathodically protecting surfaces exposed to a corrosive environment utilizing a predetermined number of anodes, each anode being connected to a power source adapted to supply a predetermined polarizing current thereto, and each anode being connected to a current controller adapted to supply an adjustingly, controlled polarizing current thereto. The polarizing current supplied by the current controller to any one anode is adjustingly controlled in response to the anode potential of that one anode with respect to the protected surface, the cathodic protection system being adapted to sequentially provide electrical continuity between each anode and the current controller and between the remaining anodes and the power source in such a manner that polarization of the system is obtained and maintained utilizing a minimum overall power consumption and in a manner maintaining a predetermined polarization over substantially the entire area of the protected surface.

METHOD AND APPARATUS FOR CATHODIC PROTECTION [75] Inventors: Olen L.Riggs, ,lr., Houston; David W. Barnett, Chute, both of Tex.

[73] Assignee: Continental Oil Company, Ponca City, Okla.

[22] Filed: Dec. 2, 1970 [21] Appl. No.: 94,353

[52] US. Cl ..204/147, 204/196 [51] int. Cl. ..C23l 13/00 [58] Field ofSearch ..204/147, 196

[5 6] References Cited UNITED STATES PATENTS 2,759,887 8/1956 Miles..204/196 2,998,371 8/1961 Sabins ..204/196 3,049,479 8/1962 Preiser etal 1 ..204/147 3,346,471 10/1967 Foroulis 204/196 3,483,101 l2/l969Delahuntetal. ..204/196 Primary Examiner-T. Tung Att0rney.loseph C.Kotarski, Henry H. l-luth, Robert B. Coleman, Jr., David H. Hill andCraig, Antonelli, Stewart & Hill Jan. 30, 1973 [57] ABSTRACT 'Anirnproved apparatus and method for cathodically W protecting surfacesexposed to a corrosive environment utilizing a predetermined number ofanodes, each anode being connected to a power source adapted to supply apredetermined polarizing current thereto, and each anode being connectedto a current controller adapted to supply an adjustingly, controlledpolarizing current thereto. The polarizing current supplied by thecurrent controller to any one anode is adjustingly controlled inresponse to the anode potential of that one anode with respect to theprotected surface, the cathodic protection system being adapted tosequentially provide electrical continuity between each anode and thecurrent controller and between the remaining anodes and the power sourcein such a manner that polarization of the system is obtained andmaintained utilizing a minimum overall power consumption and in a mannermaintaining a'predetermined polarization over substantially the entirearea of the protected surface.

7 Claims, 3 Drawing Figures POM/5E 1 SUPP!- Y l I p I wees/v7- cat/r204454 l L 43 I 52 l i 44 EJI A COMP/184702 I 50 L If 47 N 5@ METHOD ANDAPPARATUS FOR CATHODIC PROTECTION BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates generally to improvements incathodic protection systems and,.more particularly, but not by way oflimitation, to a cathodic protection apparatus utilizing a predeterminednumber of anodes wherein the polarizing current is supplied to eachanode sequentially by a power source and a current controller.

2. Description of the Prior Art In the past, cathodic protectionapparatus has been proposed wherein the current or power supply to thepower electrode or, in other words, the anode, was controlled to someextent via a reference half-cell, control circuitry. In some of thesystems of a nature mentioned above, the power supply to the anode wasplaced or switched to an on or an off position as a function ofinformation supplied via the reference half-cell. In some other systemsof this general nature, the polarizing current was adjusted to somedegree and in some manner as a function of the information supplied viathe reference electrode.

In the past, the above mentioned systems have provided adequatesolutions to various problems in the art of cathodic protection,however, it has been found that when attempting to protect particularlylarge or geometrically complex structures, most of these systems havebeen proven inadequate from'an overall protection, control apparatusinvestment, and operating cost viewpoint. The cathodic protectionsystems proposed in the past have generally been found to utilize orconsume a large total amount of power to obtain and maintain aparticular polarization, and in some other instances these cathodicprotection systems have been found inadequate toprovide a desiredprotection over the entire area of the surface being protected.

SUMMARY OF THE INVENTION The present invention contemplates a cathodicprotection apparatus for obtaining and maintaining a predeterminedpolarization of a protected surface in a corrosive" environment, numberof anodes adapted to cooperate with the protected surface which ismaintained cathodic, in such a manner that the predeterminedpolarization is obtained and maintained utilizing a minimum total powerconsumption and such that a predetermined polarization is establishedand maintained over substantially the entire area of the protectedsurface. Each anode is disposed at a predetermined position with respectto the protected surface, and each anode is adapted to cooperatinglyestablish 'an anode potential between each anode and the protectedsurface. A power supply is connected to each anode and is adapted toprovide a predetermined polarizing current to the anodes connectedthereto. The cathodic protection apparatus includes a potential controlwhich is adapted to selectively indicate the anode potential of eachanode, and to provide an output control signal proportional to one ofthe anode potentials compared to a predetermined set potential. Acurrent controller is connected to each anode and is adapted to providean adjustingly, controlled polarizing current to each anode connectedthereto. The current controller is also connected to a portion of thepotential control, and is adapted to receive the output control signalfrom the potential control. The current controller adjustingly controlsthe polarizing current provided by the current controller in response tothe output signal of the potential control. A switching control isinterposed between each anode and the power supply and between eachanode and the current controller.

The switching control is adapted to selectively and having apredetermined sequentially provide electrical continuity between one ofthe anodes and the current controller and electrical continuity betweenthe remaining of the anodes and the power supply, such thatsequentially, the current controller provides controlled polarizingcurrent to one of the anodes and the power supply provides polarizingcurrent to the remaining anodes.

An object of the invention is to provide acathodic A further object ofthe invention is to provide acathodic protection'apparatus forprotecting relatively large surface areas exposed to a corrosiveenvironment.

A still further object of the invention is to provide a cathodicprotection apparatus for protecting surfaces exposed to a corrosiveenvironment wherein the protected surface has a relatively complexgeometrical shape.

One other object of the invention is to provide a cathodic protectionapparatus for protecting surfaces exposed to a corrosive environmentwhich is economical in construction and operation.

Other objects and advantages of the invention will be evident from thefollowing detailed description when read in conjunction with theaccompanying drawings which illustrate various embodiments of theinvention.

A BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic-diagrammaticalview of a cathodic protection apparatus constructed in accordance withthe invention.

FIG. 2 is a schematicdiagrammatical view of a modified cathodicprotection apparatus, similar to the cathodic protection apparatus ofFIG. 1.

FIG. 3 is a graph showing Current Density (MA/CM) versus Time (minutes)relating to a particular test specimen cathodically protected inaccordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings ingeneral and to FIG. 1 in particular, shown schematically anddiagrammatically therein is a cathodic protection apparatus or system 10adapted to cathodically protect a surface, designated in FIG. 1 by thegeneral reference 12, and referred to below as the protected surface 12.An electro-chemical type of cathodic protection basically relates to asystem whereby an electric current is supplied to an anode emersed in anelectrolyte and to a surface which is to be protected, wherein theprotected surface potential is maintained cathodic, thereby reducing thecorrosive effect of the electrolyte on the protected surface. Cathodicprotection of this general type is well known in the art, and a detaileddescription thereof is not necessary herein.

More particularly, the cathodic protection apparatus is adapted toobtain and maintain polarization within a predetermined range oversubstantially the entire area of the protected surface 12 in contactwith a corrosive fluid or environment, referred to sometimes below as anelectrolyte and designated in FIG. 1 by the general reference 14.

The term polarization is known in the art generally as a conditionwherein corrosion is controlled by use of an electric current to opposethe electrolytic tendency of a metal to corrode (replace lostelectrons).

It should be particularly noted that the protected surface 12 has beenshown diagrammatically in FIG. 1 as having a circular shape merely forillustrative purposes, and for the purpose of clarity of description. Itshould therefore, be noted that the cathodic protection apparatus 10 ofthe present invention is particularly useful for cathodically protectinglarge surface areas, surfaces having an irregular geometricalconfiguration or, in general, surfaces wherein the anode potentialrequired to maintain a predetermined current density varies with respectto different positions on the protected surface 12, as will be madeapparent below.

The cathodic protection apparatus 10 includes a predetermined number ofpower electrodes and, more particularly, as shown in FIG. 1, two powerelectrodes 16 and 18, referred to below as the anodes l6 and 18, aredisposed in the electrolyte 14. It should be noted, that in a particularapplication utilizing the cathodic protection system of the presentinvention, more anodes may be required, and the two anodes 16 and 18 areshown in FIG. 1 merely for illustrative purposes. The factors consideredin determining the precise number of anodes to be utilized in aparticular application will be describedmore fully below.

The anodes 16 and 18 are spaced a predetermined distance apart or, moreparticularly, the anodes l6 and 18 are disposed at predeterminedpositions with respect to the protected surface 12. The spacing of theanodes l6 and 18 over the protected surface 12 and the number of anodesutilized in a particular cathodic protection system will depend on suchconsiderations as, for example, the material to be protected, that isthe composition of the protected surface 12, the throwing power" of theanodes, the geometry or geometrical configuration of the protectedsurface, the corrosivity of the particular electrolyte 14 and thetemperature of the electrolyte 14.

It is well known in the art that an anode operating in a particularcathodic protection system has a determinable throwing power" withrespect to the particular cathodic protection system. The term throwingpower" is generally used in the art to indicate the ability ofa givenelectrode or anode to cover or to maintain and hold polarized a givensurface area in contact with a particular electrolyte or corrosiveenvironment.

In the cathodic protection apparatus 10, the anodes l6 and 18 aredisposed with respect to the protected surface 12 such that the throwingpower of each anode 16 and 18 cooperates to establish and maintain apredetermined polarization over substantially the entire area of theprotected surface 12. For example, the anodes l6 and 18 may be disposedsuch that the throwing power of the anode 16 slightly overlaps thethrowing power of the anode l8, and thus the two anodes 16 and 18cooperate to protect the entire area of the protected surface 12 incontact with the electrolyte 14.

In most instances, sufficient empirical data is available to determinethe spacing or placement of the anodes over the protected surface. Inother instances, however, it may be necessary to establish the empiricaldata with respect to a particular cathodic protection system in order todetermine the spacing in accordance with the above describedcooperation. The determination of the number of anodes and thedetermination of the spacing of the anodes in a particular application,as described above, will be made more apparent below.

The cathodic protection apparatus 10, as shown in FIG. 1, also includesa predetermined number of reference electrodes, each reference electrodebeing disposed in the electrolyte 14, and one of the referenceelectrodes being disposed and adapted to cooperate with one of theanodes 16 or 18 in the operation of the cathodic protection apparatus10. The reference electrodes are constructed of a material which islocated in the e.m.f. table at a lower or more noble position than thematerial of the protected surface 12, and may be, for example, a calomelelectrode when cooperating or being utilized in a system wherein theprotected surface 12 is a stainless steel.

More particularly, as shown in FIG. 1, two reference electrodes 20 and22 are disposed in the electrolyte 14. The reference electrode 20 isdisposed with respect to the protected surface 12 and with respect tothe anode 16 to indicate the anode potential of the anode 16 withrespect to the protected surface 12, and to cooperate to control thepolarizing current being provided to the anode 16 during one portion ofthe operation of the cathodic protection apparatus 10, as willbedescribed in more detail below. The reference electrode 22, as shownin FIG. 1, is disposed with respect to the protected surface 12 and withrespect to the anode 18 to indicate the potential of the anode 18 withrespect to the protected surface 12, and to cooperate in the cathodicprotection apparatus 10 to control the polarizing current being providedto the anode I8 during one portion of the operation of the cathodicprotection apparatus 10, as will be described in more detail below.

Each anode 16 and 18 is electrically connected to the positive side of apower source or, more particularly as shown in FIG. 1, a central powersupply 24 via a two-position switch 26. The positions of the switch 26are designated in FIG. 1 as A and B and, as shown in FIG. 1, the switch26 has been positioned in the A position thereof.

The anode 16 is connected to the B-position of the switch 26 via aconductor 28, and the anode 18 is connected to the A-position of theswitch 26 via a conductor 30. The switch 26 is connected to the positiveside of the central power supply 24 via a conductor 32.

In the A-position of the switch 26, as shown in FIG. 1, the switch 26functions to provide electrical continuity between the anode 18 and thepositive output side of the central power supply 24 via the conductors36 and 32. In this position of the switch 26, there is no electricalcontinuity between the anode 16 and the central power supply 24, forreasons to be made more apparent below.

It is apparent from FIG. 1, that when the switch 26 is moved orpositioned in the B-position thereof, the switch 26 will establishelectrical continuity between the positive side of the central powersupply 24 and the anode 16 via the conductors 28 and 32, and the anode16 will not be in electrical communication with the central power supply24, for reasons which will be made apparent below. The central powersupply 24, is adapted to provide a predetermined amount of polarizingcurrent to the anode 16 or 18 connected thereto or, in other words, inelectrical continuity therewith. It is apparent from the above, that thepower supply 24 will more particularly provide polarizing current to theanode 16 or 18, depending upon the position of the switch 26. Powersupplies of this general nature are well known in the art and no furtherdescription is required herein.

Each anode 16 and 18 is also electrically connected to the positiveoutput side of an automatic current controller 34 via a two-positionswitch 36. The two positions of the switch 36 are designated in FIG. 1as A and B and, as shown in FIG. 1, the switch 36 has been moved orswitched to the A-position thereof. As shown in FIG. 1, the anode 16 iselectrically connected to the A-position of the switch 36 via aconductor 38, and the anode 18 is electrically connected to theB-position of the switch 36 via a conductor 40. The switch 36 iselectrically connected to the positive output side of the automaticcurrent controller 34 via a conductor 42.

It is apparent from FIG. 1, that in the A-position of the switch 36, asshown in FIG. 1, the switch 36 functions to provide electricalcontinuity between the anode 16 and the positive output side of theautomatic current controller 34 via the conductors 38 and 42. In thisposition of the switch 36, the anode 18 is not in electricalcommunication with the automatic current controller 34. i

It is also apparent from FIG. 1, that when the switch 36 has been movedto the B-position thereof, the switch 36 will function to provideelectrical continuity between the anode 18 and'the automatic currentcontroller 34 via the conductors 40 and 42. In the B-position of theswitch 36, the anode 16 will not be in electrical communication withautomatic current controller 34, for reasons which will be made moreapparent below.

The automatic current controller 34 is adapted to provide anadjustingly, controlled polarizing current to the anode connectedthereto or, in other words, to the anode 16 or 18 in electricalcontinuity therewith. More particularly, the polarizing current providedby the automatic current controller 34 is adjustingly controlled inresponse to a control signal received by the automatic currentcontroller 34, in a manner to be more fully described below. Currentcontrollers adapted to provide an adjustingly controlled current outputin response to a control signal are well known in the art and thereforea detailed description is not required herein.

As shown in FIG. 1, the negative side of the automatic currentcontroller 34 is electrically connected to the protected surface 12 viaa conductor 43, and the negative side of the central power supply 24 isalso electrically connected to the protected surface 12 via a conductor45 which is connected to the conductor 43. The central power supply 24and the automatic current controller 34 are thus connected to theprotected surface 12 in such a manner as to maintain the protectedsurface 12 cathodic.

As shown in FIG. 1, each reference electrode 20 and 22 is in electricalcommunication with a comparator 47 via a two-positioned switch 44. Thetwo positions of the switch 44 are designated in FIG. 1 as A and B and,as shown in FIG. 1, the switch 44 is in the A-position thereof.

The reference electrode 20 is electrically connected to the A-positionof the switch 44 via a conductor 46 and the reference electrode 22 iselectrically connected to the B-position of the switch 44 via aconductor 48. The switch 44 is electrically connected to the comparator47 via a conductor 50.

It is apparent from the foregoing in FIG. 1, that in the A-position ofthe switch 44, as shown in FIG. 1, the switch 44 functions to provideelectrical continuity between the reference electrode 21) and thecomparator 47 via the conductors 46 and 50. In this position of theswitch 44, the reference electrode 22 is not in electrical communicationwith the comparator 47.

It is also apparent from FIG. 1, that in the B-position of the switch44, the switch 44 functions to provide electrical continuity between thereference electrode 22 and the comparator 47 via the conductors 48 and50. In the B-position of the switch 44, the reference electrode 20 isnot in electrical communication with the comparator 47, for reasonswhich will become apparent below.

As mentioned before, the reference electrodes 20 and 22 are each adaptedand positioned to indicate the anode potential of one of the anodes 16or 16, more particularly, the reference electrodes 20 and 22 are adaptedand positioned such that the potential of each reference electrode 20and 22 with respect to the protected surface 12 is indicative of thepotential of the anode 16 or 16 with respect to the protected surface12. The comparator 47 is thus adapted to compare the potential of one ofthe reference electrodes 20 or 22, depending upon the position of theswitch 44 with a predetermined set potential, and to produce an outputcontrol signal 52 which is proportional to such comparison or, in otherwords, indicative of such comparison.

The automatic current controller 34 is adapted to receive the outputcontrol signal 52 from the comparator 47 as indicated in FIG. 1, and toadjustingly control the polarizing current provided by the automaticcurrent controller 34 in response to the output control signal 52.

In actual practice, the comparator 47 and the automatic currentcontroller 34 may, in some instances, comprise a unitary current controlunit which is commonly referred to in the art as a potentiastat."Potentiastats adapted to automatically adjust the current outputtherefrom in response to a particular reference or control signalsupplied thereto are well known in the art and further detaileddescription is not required herein.

As diagrammatically indicated in FIG. 1, the switches 25, 36 and 44 areinterconnected or ganged in such a manner that the switches 26, 36 and44 are each simultaneously positioned in the respective A-position or inthe respective B-position thereof. Also as shown in FIG. 1, the switches26, 36 and 44, in a preferred form, comprise a unitary switching controlunit, indicated in FIG. 1 by the general reference 56. From theforegoing, it is apparent that the switching control unit 56 isinterposed between each anode 16 and 18 and the power supply 24, betweeneach anode 16 and 18 and the automatic current controller 34, andbetween each reference electrode 20 and 22 and the comparator 47. Theswitching control unit 56 is particularly adapted to selectively andsequentially provide, for a predetermined period of time, electricalcontinuity between selected ones of the anodes and the automatic currentcontroller 34 and electrical continuity between the remaining anodes andthe power supply 24, such that sequentially the automatic currentcontroller 34 provides controlled polarizing current to some of theanodes and the power supply 24 provides polarizing current to theremaining anodes, as will be described more fully below. In other wordsand in a preferred form, the switching control unit 56 is adapted tosequentially switch or move each switch 26, 36 and 44 to the respectiveA-position or B-position thereof at predetermined time intervals, forreasons which will become more apparent below.

The switching control unit 56 may, for example, be adapted to maintainthe switches 26, 36 and 44 in the A-position for a preselected period oftwo minutes and at the end of that time move the switches 26, 36, and 44to the B-position thereof. The switching control unit 56, for example,might then hold the switches 26, 36 and 44 in the B-positions thereoffor a period of 2 minutes and at the end of that time move the switches26, 36 and 44 back to the A-positions thereof. The particular timeinterval or switching period established for the switching control unit56 with respect to a particular cathodic protection apparatus willdepend to some extent on the size of the surface area being protected bythe cathodic protection apparatus 10, the required polarization, thenumber of anodes utilized in the particular cathodic protection system,and the composition and other parameters of the electrolyte 14. In viewof the detailed description of the cathodic protection apparatus 10contained herein, the determination of the switching period for aparticular application will be apparent to those skilled in the art.

It should be specifically noted that the switches 26, 36 and 44 havebeen diagrammatically shown in FIG. 1 as being mechanically operatedmerely for the purpose of clarity of description and, in a preferredform, the switches 26, 36 and 44 would be adapted such that they wouldbe electrically or electronically operated. Electrical or electronicswitching units adapted to function in a manner as generally describedabove and as will be more particularly described below are well known inthe art and further detailed description is not required herein.

It should also be noted that there are various systems and apparatusavailable which are adapted to indicate the anode potential of aparticular anode in a cathodic protection system, and to subsequentlyprovide an output signal proportional to a comparison of that anodepotential with a predetermined set potential. The utilization of thereference electrodes 20 and 22 and the comparator 47, as shown in FIG.1, is indicative of a preferred form. Therefore, the comparator 47 andthe reference electrodes 20 and 22 are sometimes referred to below as apotential control.

OPERATION OF FIG. 1

As mentioned before, the cathodic protection apparatus 10, as shown inFIG. 1, is adapted to cathodically protect the surface 12 utilizing aminimum total power consumption, and in a manner maintainingpolarization within a predetermined range over substantially the entirearea of the protected surface 12.

During the initial start-up of the cathodic protection apparatus 10,shown in FIG. 1, the switching control unit 56 is in, what may bereferred to as an initial switching position, wherein the switches 26,36 and 44 are in the A-position. In this position, the anode 18 isreceiving polarizing current from the central power supply 24 via theswitch 26.

The power supply 24 is particularly adapted to supply a minimumpolarizing current to any particular electrode and, more particularly,as shown in FIG. 1, to provide a minimum polarizing current to the anode18 in the A-position of the switches 26, 36 and 44. The polarizingcurrent provided by the power supply 24 should be sufficient to maintainthe potential of the anode 18 within a predeterminedminimum-polarization range for a particular application such that theautomatic current controller 34 can provide sufficient polarizingcurrent to the anode 18 to bring the overall potential of the anode 18up to a predetermined potential during the time period when the switches26, 36 and 44 are moved to the B-position.

During the initial start-up and in the A-position of the switchingcontrol unit 56, the anode 16 is being provided polarizing current bythe automatic current controller 34 via the switch 36, and the referenceelectrode 20 is in electrical communication with the comparator 47. Inthis position, the reference electrode 20 will of course be registeringor measuring a minimum or, more particularly, a zero potential. Thus, inthis position, the output control signal 52 of the comparator 47 willindicate to the automatic current controller 34 that a maximumpolarizing current is to be supplied to the anode 16 via the switch 36.

It should be noted that the initial current to establish polarization inthe cathodic protection system 10 is generally higher than thepolarizing current required to maintain polarization. Therefore, thepolarizing current output of the automatic current controller 34 to theanode 16 will be at a maximum during the initial start-up of thecathodic protection apparatus 10. It should also be noted that thereference electrode 20 is continuously providing the anode potentialindication to the comparator 47 and the control signal output 52 of thecomparator 47 will thus vary in response thereto,

thereby varying the polarizing current output of the automatic currentcontroller 34 in accordance therewith.

After a predetermined period of time, the switching control unit 56 willmove the switches 26, 36 and 44 to the B-position thereof. In theB-position of the switches 26, 36 and 44, the central power supply 24will be providing polarizing current to the anode 16 via the switch 26and the automatic current controller 34 will be providing polarizingcurrent to anode 18 via the switch 36. The polarizing current beingprovided by the automatic current controller 34 to the anode 18 will beadjustingly, controlled in response to output of control signal 52 ofthe comparator 47 cooperating with the reference electrode 22, in amanner similar to that described before with respect to the anode l6 andthe reference electrode 20.

After the predetermined polarization of the cathodic protectionapparatus has been established, the power supply 24 and the automaticcurrent controller 34 will cooperate to maintain the cathodic protectionapparatus 10 within a predetermined polarization range, or in otherwords to cathodically protect the surface 12, in a manner similar tothat described above with respect to the start-up operation of thecathodic protection apparatus 10. Thus, during the operation of thecathodic protection apparatus 10, when the switches 26, 36 and 44 are inthe A-position, the power supply 24 will be providing a minimumpolarizing current to the anode 18 and the automatic current controller34 will be providing an adjustingly, controlled polarizing current tothe anode 16, that is adjustingly controlled with reference to theoutput control signal 52 of the comparator 47 cooperating with thereference electrode 20. In the B-position of the switches 26, 36 and 44of the switching control unit 56, the power supply 24 will be providinga minimum polarizing current to the anode 16 and the automatic currentcontroller 34 will be providing an adjustingly, controlled polarizingcurrent to the anode 18, that is ad justingly controlled with referenceto the output control signal 52 of the comparator 47 cooperating withthe reference electrode 22.

It is apparent from the foregoing and from FIG. 1, that the referenceelectrode functions to provide a reference anode potential'measurement'to the comparator 47 when the switches 26, 36 and 44 are in theA-position thereof, and the reference electrode 22 functions to providea reference anode potential measurement to the comparator 47 when theswitches 26, 36 and 44 are in the B-position thereof. Thus the switch 44sequentially and selectively provides electrical continuity between thecomparator 47 and the reference electrode 20 or 22.

It has been found that in cathodic protection systems utilizing only onepower anode and one reference electrode, that the power anode mayconsume substantially more total power output than necessary to maintainthe particular desired, polarization. It has also been found that, insome instances, utilizing a single power anode and a single referenceelectrode that the predetermined potential at which the power anode ismaintained may be insufficient to provide the cathodic protectionrequired by the system or, in other words, to maintain the particularcathodic protection system within a predetermined polarization. This ofcourse results in an overall increased operating cost and, in someinstances, an increased corrosion rate with respect to particular areasof the protected surface.

The utilization of a predetermined number of power electrodes or anodesand a predetermined number of cooperating reference electrodes, in amanner as described above, permits the entire cathodic protection systemto be maintained within a predetermined range utilizing less total powerconsumption and assures that all areas of the protected surface aremaintained within that polarization range, thereby effectively reducingand controlling corrosion.

The above is particularly important when considering the utilization ofa cathodic protection apparatus to protect tanks, having a relativelylarge surface area in contact with a corrosive fluid, wherein thevarious parameters of the corrosive fluid in the tank may significantlyvary throughout the tank due to the large volumetric area occupied bythe corrosive fluid. Thus, it is apparent from the foregoing that insuch a tank, for example, the temperature of the corrosive fluid or thecorrosive environment could vary considerably from one portion of thetank with respect to another portion of the tank, thus requiring morepolarizing current to be provided at one area and less polarizingcurrent to be provided at another area.

The cathodic protection apparatus 10 of .the present invention thusestablishes and maintains a predetermined polarization over the entirearea protected surface 12 in such a manner that the polarizing currentto each predetermined area of the protected surface 12 is maintained ata minimum and yet is sufficient to cathodically protect that portion ofthe protected surface 12. It should be particularly noted that the termtotal power consumption of the cathodic protection apparatus refers morespecifically to the power consumed by the central power supply 24 andthe automatic current controller 34 in establishing and maintaining thepredetermined polarization.

It should also be noted that the utilization of a predetermined numberof power anodes and predetermined number of associated referenceelectrodes cooperating therewith, in a manner as described above,becomes important in attempting to cathodicallyprotect surfaces havingan irregular or relatively complex geometrical shape. The variousparameters of the corrosive fluid, in such an instance, and thepolarizing current required to establish and maintain particular areasof the surface within a predetermined polarization range may varyconsiderably.

The graph shown in FIG. 3 is a plot of current density, measured inmilliamps per centimeter squared, versus time, measured in minutes, fora particular experimental utilization of the cathodic protectionapparatus 10, as described before. In this particular experiment, thetest specimen was annealed I020 carbon steel with a surface area of 22square centimeters. The electrolyte utilized was a merchant grade H PO,percent by weight) at a temperature of 24 centigrade. The referenceelectrode was a saturated calomel cell (SCE). The time interval or timeperiod for the switching control unit 56 was set to sequentially switchthe switches 26, 36, and 44 between the A-position and the B-position at2-minute time intervals.

It is apparent from FIG. 3 that the current density required to maintainpolarization within a predetermined range decreased over a period oftime, the larger amount of polarization current being required toinitially establish polarization. More particular, and as shown in FIG.3, in approximately 120 minutes the current density required to maintainthe test specimen (the carbon steel) cathodically protected wasapproximately 0.1 milliamps per square centimeter, whereas the currentdensity required to establish polarization varied from approximately 2.0to 0.15 milliamps per square centimeter in approximately 60 minutes. Inthis example, the corrosion rate was determined to be approximately l.2mpg, whereas normally the corrosion for unprotected steel is between 500and 600 mpg.

From the foregoing it is apparent that the total polarizing current,that is the current provided by the central power supply 24 and theautomatic current controller 34, to maintain a predeterminedpolarization decreased over a period of time. Thus, in this example, thetotal power consumed by the cathodic protection system was substantiallyreduced over a period of time, since the current density required tomaintain the predetermined polarization decreased during that period oftime.

It should also be noted that the switching control unit 56 cooperateswith the central power supply 24 and the automatic current controller34, as described above, in a manner such that a large number of anodesare effectively and efficiently controlled by the single unitcombination. Thus, the cathode protection apparatus 10 is also adaptedto control a large number of anodes utilizing a reduced number ofprotection units.

DESCRIPTION OF FIG. 2

The cathodic protection apparatus 10a, shown in FIG. 2, is constructedsimilar to the cathodic protection apparatus 10, described before. Oneof the differences between the cathodic protection apparatus 100 and thecathodic protection apparatus 10 is that each switch 26a, 36a, and 44aof the switching control unit 56a is a three-positioned switch, having athird position designated in FIG. 2 by the letter or switch position C.

As shown in FIG. 2, when the switching control unit 56a has beenactuated to move the switches 26a, 36a, and 44a to the C-positionthereof, the central power supply 24 and the automatic currentcontroller 34 are not in electrical communication with either the anode16 nor the anode 18. In other words, the C-position of the switchingcontrol unit 560 represents a switch position wherein polarizing currentis not being provided via the central power supply 24 nor the automaticcurrent controller 34.

As shown in FIG. 2, a programmer 70 is disposed between the switchingcontrol unit 56a and the protected surface 12. More particularly, and asillustrated in FIG. 2, the programmer 70 is adapted to sense variousparameters relating to the protected surface 12 or the electrolyte 14via a sensing conductor 72 and to provide an actuating control signal 74to the switching control unit 56a. The control signal 74 is responsiveto the particular sensing control signal 72 being supplied to theprogrammer 70, as will be made more apparent below.

As diagrammatically indicated in FIG. 2 and in a preferred form, theprogrammer is adapted to control the switching sequence of the switchingcontrol unit 560. More particularly, the programmer is adapted tocontrol the cycle time or period of the switching control unit 560, thatis the particular period of time in which the switching control unit 56awill maintain the switches 26a, 36a, and 44a in the A-position, the B-position or the C-position thereof.

Since the particular amount of polarizing current required to beprovided to a particular anode at a given time and in a particularcathodic protection system to maintain the system polarized depends uponsuch parameters as, for example, the temperature of the electrolyte 14,as mentioned before. The programmer 70, in that instance, can be moreparticularly adapted such that the sensing signal 72 is responsive tothe temperature of the electrolyte 14. The output control signal 74 ofthe programmer 70, in that example, would set the cycle time orswitching period of the switching control unit 56a in accordance withthe measured temperature of the electrolyte 14.

As shown in FIG. 2, the C-position of the switches 26a, 36a, and 44aessentially represents an of position, the sensing signal 72 might beadapted to sense the presence of the electrolyte within the protectedsurface 12 and the programmer 70 might then be adapted to cooperate withthe switching control unit 56a to move the switches 26a, 36a, and 44atherein to the A- position and to automatically begin the cyclesequencing of the switching control unit 56a, in a manner similar tothat described before.

From the foregoing it will be apparent to those skilled in the art, thatin particular cathodic protection systems, the programmer 70 could beadapted to sense any particular controlling parameter or parameters inthat system and to control the switching cycle of the switching controlunit 56a in response to such a sensing signal. It will also be apparentto those skilled in the art, that although the C-position of theswitches 26a, 36a, and 44a, as shown in FIG. 2, is essentially an off"position, that in a particular cathodic protection system, theC-position of the switching control unit 56a could represent a switchposition wherein the central power supply 24 was providing polarizingcurrent to both anodes l6 and 18.

OPERATION OF FIG. 2

The cathodic protection apparatus 10a, shown in FIG. 2, will operatesubstantially similar to the cathodic protection apparatus 10, describedbefore. One of the salient differences in the operation of the cathodicprotection apparatus 10a, is that the switching period of the switchingcontrol unit 560 is controllingly varied in response to the controlsignal 74 from the programmer 70. Since the programmer 70 is adapted toreceive a sensing signal 72 which is responsive to various parameters ofthe cathodic protection system 10a, it is apparent that the cathodicprotection apparatus 10a provides a more complete automatic control,which may be more effective or desirable in some applications.

It will be apparent from the foregoing, that the cathodic protectionapparatus 10a retains all of the advantages of the cathodic protectionapparatus 10,

described before, and yet provides an additional control feature whichmay operate in an overall cathodic protection system to still furtherreduce the total power consumption and yet maintain the predeterminedpolarization.

As mentioned before, the two anodes l6 and 18 have been shown merely forillustrative purposes and, in a particular cathodic protection system,as contemplated by the present invention, more anodes may be utilized.The precise number of additional anodes and the placement of suchadditional anodes with respect to the protected surface will be apparentto those skilled in the art from the above detailed description.

Changes may be made in the construction and arrangement of the parts orthe elements of the various embodiments as disclosed herein withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims.

What is claimed is:

l. A cathodic protection apparatus for obtaining and maintainingpolarization of a protected surface in a corrosive environment, theprotected surface being maintained cathodic, comprising:

a plurality of anode means, each anode means being disposed at apredetermined position with respect to the protected surface;

a current controller means adapted to selectively provide an adjustinglycontrolled, polarizing current to one of said anode means;

a power supply means adapted'to selectively provide a predeterminedpolarizing current to the remaining of said anode means;

a reference electrode means mounted in the vicinity of each of saidanode means and generating an electrical control signal corresponding tothe passivation condition of said protected surface, said currentcontroller means being adapted to receive said electrical control signaland to adjustingly control the polarizing current provided by thecurrent controller in response thereto; and

a switch means in electrical communication with said plurality of anodemeans, said power supply means, said current controller means and saidreference electrode means, whereby said switch means is adapted toselectively and sequentially provide electrical continuity between saidremaining anode means and said power supply means and to provideelectrical continuity between said one anode means, said referenceelectrode means in the vicinity of said one anode means and said currentcontroller means.

2. A system of claim 1 in which the reference electrode means is locatedwithin the area of the throwing power of its corresponding anode meansfor which the passivation condition is being measured.

3. A system of claim 1 in which the plurality of anode means, controllermeans and switch means are placed and cooperate such that thepassivation condition encompasses substantially the entire area of thesurface being protected in the corrosive environment.

4. A system of claim 1 in which the controller means and switching meansare adapted to operate in a cyclic manner with the period of time foreach step in the cycle being adjusted according to a predeterminedprogngnA system of claim 1 in which the controller means and switchingmeans are adapted to operate in a cyclic manner with the period of timefor each step in the cycle and the overall cycle being adjusted inresponse to a control signal indicative of parameters of the passivationsystem.

6. The apparatus of claim 1 which is adapted to maintain a minimumoverall power consumption and passivation condition for a large complexsurface in a corrosive environment.

7. A method of protecting a surface by obtaining and maintainingpolarization of the surface in a corrosive environment to maintain thesurface cathodic with a minimum overall power consumption andpassivation condition comprising:

disposing a plurality of anode means at a predetermined position withrespect to the protected surface;

adapting a current controller means to selectively and adjustinglyprovide and control a polarizing current to one of said anode means;

adapting a power supply means to selectively provide a predeterminedpolarizing current to the remaining of said anode means;

mounting a reference electrode means in the vicinity of each of saidanode means and generating an electrical control signal corresponding tothe passivation condition of said protected surface;

adapting said current controller means to receive said electricalcontrol signal and to adjustingly control the polarizing currentprovided by the current controller means in response thereto; and

connecting by a switch means said plurality of anode means, said powersupply means, said current controller means and said reference electrodemeans, whereby said switch means selectively and sequentially provideselectrical continuity between said remaining anode means and said powersupply means and provides electrical-continuity between said one anodemeans, said reference electrode means in the vicinity of said one anodemeans and said current controller means.

1. A cathodic protection apparatus for obtaining and maintainingpolarization of a protected surface in a corrosive environment, theprotected surface being maintained cathodic, comprising: a plurality ofanode means, each anode means being disposed at a predetermined positionwith respect to the protected surface; a current controller meansadapted to selectively provide an adjustingly controlled, polarizingcurrent to one of said anode means; a power supply means adapted toselectively provide a predetermined polarizing current to the remainingof said anode means; a reference electrode means mounted in the vicinityof each of said anode means and generating an electrical control signalcorresponding to the passivation condition of said protected surface,said current controller means being adapted to receive said electricalcontrol signal and to adjustingly control the polarizing currentprovided by the current controller in response thereto; and a switchmeans in electrical communication with said plurality of anode means,said power supply means, said current controller means and saidreference electrode means, whereby said switch means is adapted toselectively and sequentially provide electrical continuity between saidremaining anode means and said power supply means and to provideelectrical continuity between said one anode means, said referenceelectrode means in the vicinity of said one anode means and said currentcontroller means.
 2. A system of claim 1 in which the referenceelectrode means is located within the area of the throwing power of itscorresponding anode means for which the passivation condition is beingmeasured.
 3. A system of claim 1 in which the plurality of anode means,controller means and switch means are placed and cooperate such that thepassivation condition encompasses substantially the entire area of thesurface being protected in the corrosive environment.
 4. A system ofclaim 1 in which the controller means and switching means are adapted tooperate in a cyclic manner with the period of time for each step in thecycle being adjusted according to a predetermined program.
 5. A systemof claim 1 in which the controller means and switching means are adaptedto operate in a cyclic manner with the period of time for each step inthe cycle and the overall cycle being adjusted in response to a controlsignal indicative of parameters of the passivation system.
 6. Theapparatus of claim 1 which is adapted to maintain a minimum overallpower consumption and passivation condition for a large complex surfacein a corrosive environment.